As a general rule in processes such as those for fluid cat crackers it has been common practice to employ fully-shrouded trickle valves on all of the primary and secondary cyclone diplegs for sealing the legs in order to insure good cyclone operation. These valves generally are located in the fluid beds such that if the valve fails, a hydraulic seal will result. That is, the solids will be directly discharged into the bed and solids can be sucked up into the dipleg from the bed to establish a level of catalyst in the dipleg which will correspond to the pressure in the dipleg. This pressure is generally lower than the pressure at the top of the bed so that the level will be higher than the bed level. To prevent action of the bubbles which occur in the fluid bed from affecting the flapper action (i.e., opening and closing motion) of the valve, it is common to employ a large pipe shroud around the valve. Valves which employ such expedients are commonly referred to as fully shrouded valves such as disclosed in U.S. Pat. No. 2,838,062. Other aspects and applications of these conventional valves may be found by referring to U.S. Pat. Nos. 2,838,065; 2,901,331; and 2,838,063, the latter of which is considered most relevant to the present invention. Alternatively, oftentimes splash baffles are used on the cyclone diplegs because of the relatively large cost of the alloy trickle valves. It is desirable, however, to use these valves in order to maximize the density of the catalyst in the cyclone diplegs and to maintain the catalyst flow down the dipleg, i.e., preserve the collection efficiency of the cyclone, especially during times when the catalyst levels are below the level of the outlet from diplegs.
The use of full shrouds was developed at a time when the size of the bubbles which are developed in the fluid cat cracker beds was a relatively unknown factor. At the time it was suspected that large bubbles existed which would violently and undesirably move the flapper plate out of its desired position. In accordance with recent investigations in fluid solids technology, it is now believed that these bubbles, in the use of regenerators, are substantially smaller than originally anticipated, being about one inch in diameter, and in the case of reactors, are about three inches in diameter. It, therefore, has become apparent that the need for these large full shrouds heretofore employed no longer exists.
However, it is apparent that some protection of the flapper plate is necessary in order to prevent a local disturbance from torch oil, spray water, eroded grid holes, broken seals, etc., from affecting its position with respect to the outlet opening from the dipleg, which could result in displacing the flapper and/or diverting gas directly into the dipleg. Further incentives for eliminating the shroud are the relatively high cost of the valves which are usually made of a high cost alloy steel and, therefore, the shroud adds considerably to the valve cost. The shrouds also are extremely large, e.g., 26 inches in diameter for a typical 16 inch dipleg, and take up a large amount of the bed or cross-sectional area, which reduces the volume available for effective regeneration or reaction of catalyst. In addition, the relatively large weight of the shrouds places an undesirable bending moment on the diplegs.